Adam Clarke on Joshua 24:31

And Israel served the Lord, etc. - Though there was private idolatry among them, for they had strange gods, yet there was no public idolatry all the days of Joshua and of the elders that overlived Joshua; most of whom must have been advanced in years at the death of this great man. Hence Calmet supposes that the whole of this time might amount to about fifteen years. It has already been noted that this verse is placed by the Septuagint after Jos 24:28.